Friction stir welding is a relatively new process using a rotating tool to join two workpieces in a solid state. At present, the process is applied almost exclusively in straight line welds. For example, such a process is described in U.S. Pat. No. 5,460,317 to Thomas et al. When using friction stir welding to join two workpieces, or repair cracks in a single workpiece, one of the primary parameters that must be monitored and controlled is the force exerted by the tool on the workpieces or workpiece. The magnitude of the force exerted by the tool must be maintained above a prescribed minimum in order to generate the required frictional heating of the workpieces. When joining two workpieces along a straight line, the tool is plunged by the friction stir welding machine to the depth necessary to produce the required resistance force to generate the frictional heating. Once that depth is achieved no further monitoring is required provided that proper depth is maintained.
As illustrated in FIGS. 8A and 8B, when joining workpieces 10, 11 having complex, curvilinear geometries, controlling the depth of the tool 9 becomes problematic because the complex geometry 12 may not perfectly match the contoured path 13 that the numerically controlled machine is programmed to follow. If a discrepancy .delta. between the actual workpiece geometry and the programmed path occurs, the weld joint formed may be defective as the friction stir welding tool will either exceed the optimum depth or else fail to sufficiently penetrate the workpieces. This discrepancy may occur as a result of tolerance build-up during the fabrication of the workpieces being joined, as well as a possible error in the set-up of the workpieces on the friction stir welding machine or other manufacturing or engineering errors.